1. Field of the Invention
The present invention is related to a thermal management system for high-temperature fuel cell. Especially, it refers to a thermal energy circulation mechanism for fuel cell to reduce installation cost, minimize fuel consumption and pollution, and improve overall operation efficiency.
2. Description of the Prior Art
In principle, fuel cell is operated on cathode and anode that are filled with electrolyte solutions. Between the two electrodes, there is a permeative membrane. Hydrogen enters the cell from anode while oxygen (or air) enters from cathode. With catalyst, hydrogen atom at anode is dissociated to two protons and two electrons. The protons are attracted by oxygen and move to the other side of the membrane. The electrons flow through the external circuit and arrive in cathode. With the catalyst at cathode, hydrogen protons, oxygen and electrons react to form water molecules. Thus, the only product of fuel cell (emission) is water. The above “hydrogen” fuel can come from any hydrocarbons, such as natural gas, methanol, ethanol (alcohol), water electrolysis, biogas . . . etc. Since fuel cell utilizes the chemical reaction between hydrogen and oxygen to produce electricity and water, there is absolutely no pollution and no issues with traditional battery in lengthy charging process. Moreover, fuel cell has become a generally recognized alternative to fossil fuels due to its low cost, wide fuel selection (including: pure hydrogen, methanol, ethanol and natural gas etc.), no hazards in reaction process and potential use of by-product (water).
Among various types of fuel cell structures, high-temperature fuel cell (such as solid oxide fuel cell SOFC and molten carbonate fuel cell etc.) must operate at high temperature. Thus, it is a feasible and common practice to recover the thermal energy from the tailpipe emission that contains high heat content and to heat incoming gas. Since heat is generated during the power generation process of fuel cell, excessive heat causes excessively high temperature or temperature increase rate for fuel cell. To prevent damage to fuel cell due to high temperature, it usually needs to introduce a large amount of air to maintain a stable temperature. However, the introduced air at cathode contains more oxygen than that is needed by fuel cell. It will reduce the loss of thermal energy and improve overall power generation efficiency by recycling most of the hot air at cathode and adding necessary fresh air.
The thermal energy cycle in a traditional high-temperature fuel cell is shown in FIG. 1. Mainly, a fuel blower 12 withdraws fuel from a fuel tank 11 and transfers the gasified fuel to the first mixer 10. A water pump 52 transfers the water from a water tank 51 to the third heat exchanger 50 that converts water into steam. Then the steam is delivered to the first mixer 10 and mixes with the fuel. The mixed fuel steam is sent to the first heat exchanger 6 for heating and then to a reformer 3. The reformer 3 will adjust the ratio of gas fuel to steam and outputs to the anode input 4Ai of fuel cell 4. An air blower 202 sends external air 21 to the second heat exchanger 20 for heating and then to the cathode input 4Ci of fuel cell 4. In the above structure, the cathode output 4Co of fuel cell 4 connects to the second mixer 8 through the first heat exchanger 6 to form a cathode thermal cycle pipeline C. So after the high-temperature air produced by fuel cell 4 is used to heat the first heat exchanger 6 and then introduced to the second mixer 8. The anode output 4Ao of fuel cell 4 can directly connect to the second mixer 8 to allow the steam and heat produced from fuel cell 4 to enter the second mixer 8 and mix with previously-mentioned high-temperature air. After heating by a combustor 7, the stream passes through the reformer 3, the second heat exchanger 20 and the third heat exchanger 50 for heat exchanging (heating). At last, low-temperature water steam and residual fuel are discharged to outside. The cycle allows the high-temperature air from cathode output 4Co to enter the second mixer 8 and the steam and high-temperature residual fuel from anode output 4Ao to enter the second mixer 8. This enables recovery of thermal energy produced by fuel cell 4.
However, the above structure has the following drawbacks in a practical application:
1. Since the assembly consists of three heat exchangers and one combustor, its overall installation cost is high and its size is huge.
2. It lacks recycle mechanism for anode fuel and fails to recycle the water steam and residual fuel. Therefore, both fuel consumption and power generation efficiency are poor. Besides, the recovery of cathode hot air is poor and thermal energy efficiency cannot be improved.
3. Because a combustor is used for heating, it increases fuel consumption and the combustion emission may cause environmental pollution.
In view of the above drawbacks associated with the thermal cycle for a traditional high-temperature fuel cell, the inventor has made improvements in the present invention.